Known variable-speed drives normally comprise an input shaft, an output shaft, and a pair of conical bodies, each at least angularly integral with a respective shaft, and connected angularly to each other via the interposition of an intermediate element, the movement of which in relation to the conical bodies provides for varying the velocity ratio of the input and output shafts.
Depending on the application, the intermediate element is defined by a further rigid body, normally a cone or cylinder, or by a flexible element designed to cooperate with a lateral surface of the conical bodies.
In most applications in which a rigid body is used, at least part of the generating line of the body cooperates in sliding manner with part of the generating line of the conical bodies.
Though universally adopted, such a design presents a major functional drawback due to the variation in surface speed along the contacting portions of the generating lines resulting in relative slide between the contacting profiles, thus resulting in increased power loss due to friction and, consequently, in reduced efficiency of the variable-speed drive. Relative slide and, consequently, power loss may be reduced by reducing the length of the contacting portion of the generating line so that, at most, the rotary bodies contact at one point only.
Known design solutions for overcoming the above drawback are nevertheless unsatisfactory in terms of power transmission, which can only be increased by increasing the number of movable intermediate elements, thus resulting in an increase in the size and manufacturing cost of the drive.
The increase in cost is due to the fact that, according to the above solutions, one of the conical bodies must necessarily be axially mobile in relation to the other, thus requiring the use of self-compensating axial joints, i.e. capable of exerting predetermined axial pressure as a function of the torque on the conical drive body.
From DE-A-902 204 it is also known a solution wherein the conical bodies have conjugated curved side surfaces, which are kept in direct contact to each other by forcing means defined by an outer rigid ring axially movable in sliding contact with respect to the conical bodies. Even this solution is however not free from drawbacks, mainly consisting in the fact that the variable speed drive, in order to change the speed ratio between the input and output shafts by moving the ring axially, is to be provided with a friction device able to remove the load from the drive during the change speed operation; in this manner the drive cannot be employed as a continuous one despite the almost infinite speed ratios available.